As plasma deposition of thin films becomes an increasingly more viable process for commercial production, an understanding of those parameters which control the chemical structure and properties of the film assumes greater importance. A glow discharge system in its simplest form comprises a deposition chamber, means for evacuating the chamber, means for introducing gases into the chamber, a power source and means for efficient transfer of the power to the gases. Kaganowicz et al. show such a glow discharge system in U.S. Pat. No. 4,339,471. Glow discharge systems for commercial scale production typically include a deposition chamber wherein substrates are automatically conveyed under (or over) a series of "in-line" electrodes.
The chemical composition and properties of the deposited film are determined by the species generated in the plasma. These species in turn depend on the power applied to the electrodes and the potential between the electrodes and the substrate to be coated. Since the power supplies typically utilized in these systems have a constant power output it becomes necessary to optimize and maintain the size and spacing of the electrodes with respect to the substrate for a particular coating. In this way when a given potential is correlated to a desired film structure, reproducibility of that film is insured.
In practice, however, as much as a 30 percent drop in potential has been noted after several hours of operation which results in a change in the properties of the deposited film. It is believed the drop is caused by substantial deposition of material onto the electrodes. It is necessary then to shut down the system and remove the buildup of material on the electrodes.
It would be desirable to be able to deposit reproducible coatings by glow discharge in a production environment without having to interrupt the process every 2 to 3 hours.